The present invention relates to control systems for multiple cylinder internal combustion engines, and in particular to a control system which combines closed-loop mixture control and split engine operations.
It is known that fuel economy is achieved under light load conditions by operating a multiple cylinder engine on partial cylinders if the reduced engine power can adequately operate the vehicle. When the engine load is relatively heavy the engine is operated on full cylinders. The whole cylinders are thus divided into a first group which is deactivated at light loads and a second group which is operated at all times. This method of engine control is known as split engine operation. On the other hand, closed-loop mixture control systems are also known and widely used as an effective means of eliminating noxious gaseous components. Such systems employ an exhaust gas sensor and a three-way catalytic converter disposed downstream of the gas sensor to effect simultaneous oxidation of hydrocarbon and monoxide and reduction of nitrogen oxides when the air-fuel ratio is precisely controlled to within a narrow range, known as conversion efficiency window near stoichiometry.
However, the above known methods cannot be combined together without giving rise to a problem in that the deactivated cylinders under light load act as an air pump to introduce air into the exhaust system thereby increasing oxygen contents therein, which results in a false gas sensor signal. To prevent this problem, a prior method involves the use of a shutoff valve to direct the stream of pumped air through a passage that bypasses the catalytic converter during partial cylinder mode, and switch the direction of the gas flow to the catalytic converter during the full cylinder mode.
However, because of the inherent delay time the exhaust gas takes to reach the location of the shuftoff valve, the signal that controls the shutoff valve must be precisely timed in relation to the delay time and if improperly timed a false gas sensor would result.